1. Field of the Invention
The present invention relates to a sheet convey apparatus suitable for use with an image forming apparatus such as an ink jet recording apparatus wherein an image is formed by scanning a recording head in serial manner.
2. Related Background Art
In the past, various recording apparatuses having various image forming means have been put to practical use. Among them, ink jet recording apparatuses and thermal transfer recording apparatuses have a low manufacturing cost and are less noisy and have been widely used from personal use to office use. In image forming apparatuses of the ink jet type and thermal transfer type, generally, an image is formed by shifting to a recording sheet relative to a recording portion of the apparatus. In printers of serial scan type using the ink jet recording method, the recording sheet is intermittently shifted by a predetermined width and a portion of the image is formed on the width of the recording sheet which was shifted. Further, the recording sheet is generally conveyed by means of convey rollers disposed at upstream and downstream sides of the recording portion. In order to reduce the margin in the recording sheet, a tip end or front portion of the recording sheet in a condition that the recording sheet is pinched only between the convey rollers disposed at the upstream side of the recording portion, an intermediate or middle portion of the recording sheet in a condition that the recording sheet is pinched between both the convey rollers disposed at the upstream and downstream sides of the recording portion, and a trail end or rear portion of the recording sheet in a condition that the recording sheet is pinched only between the convey rollers disposed at the downstream side of the recording portion.
In the ink jet recording apparatuses, elongation of the recording sheet occurs at the recording portion due to ink. Thus, even when the recording sheet is conveyed by the cooperation of the upstream and downstream convey rollers, in order to convey the recording sheet accurately, a convey amount of the recording sheet must be controlled by the upstream convey rollers. Accordingly, when the recording sheet is between both the upstream and downstream convey rollers, by setting a recording sheet pinching pressure (P1) of the upstream rollers to become greater than a recording sheet pinching pressure (P2) of the downstream rollers, the conveying amount is regulated by the convey rollers disposed at the upstream of the recording portion, and, by setting the sheet convey amount (L2) of the downstream convey rollers to become greater than the sheet convey amount(L1) of the upstream convey rollers, the recording sheet is conveyed while slipping the downstream rollers, thereby preventing the looseness of the recording sheet at the recording portion.
In FIG. 11 which is a sectional view showing a main portion of a conventional recording portion, a convey roller 1 is disposed at an upstream side of the recording portion, and a driven roller 3 serves to bias a recording sheet 9 against the convey roller 1. Similarly, a sheet discharge roller 2 is disposed at a downstream side of the recording portion, and a driven roller 4 serves to bias the recording sheet 9 against the sheet discharge roller 2.
In this example, a ratio between a biasing force of the driven roller 3 associated with the convey roller and a biasing force of the driven roller 4 associated with the sheet discharge roller 2 is about 4:1. With this arrangement, even in the condition that the recording sheet 9 is pinched by both the rollers 1 and 2, the convey amount of the recording sheet 9 is governed by the convey roller 1. Further, although the convey amount of the sheet discharge roller 2 is set to be greater than the convey amount of the convey roller 1, since the pinching pressure of the sheet discharge roller is smaller than that of the convey roller, the recording sheet 9 is conveyed while being slipped, thereby preventing the floating and looseness of the recording sheet from occurring at the recording portion. Incidentally, in the condition that the recording sheet 9 is pinched only by one of the convey roller and the sheet discharge roller, the convey amount of the recording sheet 9 depends upon the associated roller.
In FIG. 11, the reference numeral 11 denotes a platen for supporting the recording sheet; and 13 denotes an ink jet recording head mounted on a carriage 14. Further, in FIG. 11, the recording sheet is shifted from right to left, and there are the following relations P1&gt;P2 and L2&gt;L1.
With the arrangement as mentioned above, when the recording sheet is conveyed, since the convey amount is determined by the upstream side convey roller 1 regarding the front and middle portions of the recording sheet, there is no problem. However, immediately after the trail end of the recording sheet leaves a nip between the upstream pair of rollers 1, 3, the convey amount of the recording sheet is governed by the sheet discharge roller 2 disposed at the downstream side of the recording portion. By the way, as mentioned above, since the convey amount of the downstream pair of rollers 2, 4 is greater than that of the upstream pair of rollers 1, 3, after the trail end of the recording sheet leaves the nip between the upstream pair of rollers, the convey amount of the recording sheet is increased, with the result that image portions are not contiguous with each other, thereby greatly deteriorating the image quality.
To avoid such a drawback, the following -trials have been proposed.
(1) The upstream and downstream convey rollers have different drive sources, the downstream convey amount L2 is kept to be greater than the upstream convey amount L1 until the trail end of the recording sheet leaves the upstream rollers, and, after the rotation of the rollers by which the trail end of the recording sheet passes through the nip between the upstream rollers, the downstream convey amount L2 is changed to L1; PA1 (2) Although the upstream and downstream convey rollers have a common drive source, the downstream rollers have a speed change mechanism, and the downstream convey amount L2 is kept to be greater than the upstream convey amount L1 until the trail end of the recording sheet leaves the upstream rollers, and, after the rotation of the rollers by which the trail end of the recording sheet passes through the nip between the upstream rollers, the downstream convey amount L2 is changed to L1; PA1 (3) Although the upstream and downstream convey rollers have a common drive source, there is no speed change mechanism, and the downstream convey amount L2 is kept to be greater than the upstream convey amount L1 till the rotation of the rollers by which the trail end of the recording sheet passes through the nip between the upstream rollers, and, after the trail end of the recording sheet leaves the upstream rollers, the downstream convey amount L2 is changed to L1 (in this case, the upstream convey amount becomes L1.times.L1/L2); and PA1 (4) Although the upstream and downstream convey rollers have a common drive source, there is no speed change mechanism, and the downstream convey amount L2 is kept greater than the upstream convey amount LI until the trail end of the recording sheet leaves the upstream rollers, and, after the rotation of the rollers by which the trail end of the recording sheet passes through the nip between the upstream rollers is reached, the downstream convey amount L2 is changed to L1 (in this case, the upstream convey amount becomes L1.times.L1/L2).
The above methods (1) to (4) are shown in FIG. 12 collectively.
Among the above methods, in the methods (1), (2) and (4), since the downstream convey amount is decreased to L1 (&lt;L2) when the recording sheet passes through the nip between the upstream rollers, the looseness or slack in the recording sheet generated due to the elongation of the sheet at the recording portion cannot be eliminated. For example, when it is assumed that the recording width X at the recording portion in a sheet convey direction is 16.256 mm and a rate k of elongation of the sheet for each recording width is 0.01, the elongation of each recording width of the sheet becomes 0.16256 mm (=16.256.times.0.01).
If the elongation of the recording sheet is generated uniformly in the front-and-rear direction of the recording portion, even when the convey amount is correct, the end position of the recording width of the sheet will be deviated from the end of the recording portion by 0.08128 mm (=0.16256/2) in the upstream direction. This deviation amount exceeds a distance of 0.0635 mm between ink jet nozzles having 400 DPI pitch, with the result that the extended recording width overlaps with a next recording width, thereby generating a high density stripe.
On the other hand, even if the elongation of the recording sheet is not generated uniformly in the front-and-rear direction of the recording portion, the recording sheet will be floating at the recording portion, which results in contact between the recording sheet and the recording head, and wrinkles in the sheet due to the fact that the floating sheet is pinched between the downstream rollers.
Further, in the above method (3), although the downstream convey amount is maintained to L2 (&gt;L1) when the trail end of the recording sheet leaves the nip between the upstream rollers, upon starting the rotation of the rollers by which the trail end of the recording sheet passes through the nip between the upstream rollers, if a distance c between the nip of the upstream rollers and the trail end of the recording sheet is almost zero, since the trail end of the recording sheet leaves the nip between the upstream rollers immediately, the downstream rollers cannot compensate for all of the elongation of the recording sheet at the recording portion.
For example, when the rate k of elongation of the sheet for each recording width is 0.01, as mentioned above, the elongation of each recording width of the sheet becomes 0.16256 mm. In order to eliminate the slack in the sheet by pulling the elongation of the sheet by means of the downstream rollers, the convey amount of the downstream rollers must be set to 16.4186 mm (=16.256+0.16256) or more.
In this case, however, during the recording immediately before the trail end of the recording sheet leaves the nip between the upstream rollers, when the distance c between the trail end of the recording sheet and the nip of the upstream rollers is 5 mm, since only the distance c contributes to the pulling operation of the downstream rollers, the slack in the sheet cannot be removed only by the elongation of 0.05 mm (=0.16256.times.c/X). Thus, the remaining elongation of 0.11256 mm (=0.16256-0.05) cannot be eliminated. Also in this case, the extended recording width overlaps with the next recording width, thereby generating the high density stripe.
As mentioned above, when the convey amount is changed from the predetermined value, if the convey amount is insufficient, the recording width which was previously recorded is partially overlapped with the next recording width which is to be recorded, thereby generating the high density stripe in the image. Further, in dependence upon the distance c between the trail end of the recording sheet and the nip of the upstream rollers, the elongation of the sheet cannot be completely eliminated even when the convey amount of the downstream rollers is not decreased (i.e. maintained to L2), thereby also generating the high density stripe.